Industrial process control system for operations with long time constants

ABSTRACT

A system for providing continuous and proportional control of a process variable includes a sensor furnishing an output signal having an analog value proportional to the process variable. An analog controller includes a shaping circuit comparing that analog value with a reference to develop an error signal, and an integrator and two differentiators in series. The time constant of the integrator is chosen to filter out fast, unwanted fluctuations in the process variable and the time constants of the differentiators to approximate those involved in the process work function. The outputs of these elements of the analog controller are summed and supplied as a control signal to an actuator control circuit which varies the power supplied to an actuator so as to proportionally control a control parameter of the process. In turn, changes in the control parameter bring the process variable back to its desired value. An embodiment of the analog controller uses tantalum capacitors and high impedance operational amplifiers. Embodiments of the actuator control circuits for use with AC synchronous stepping motors and DC motors include an integrator which eliminates any dead band in the actuator&#39;&#39;s response to the control signal.

United States Patent [72] Inventor Fritz K. Preikschat 16020 Lake HillBlvcL, Bellevue, Wash. 98004v 1211 Appl. No. 54,534 [22] Filed July 13,1970 [45] Patented Dec. 14, 1971 [54] INDUSTRIAL PROCESS CONTROL SYSTEMFOR OPERATIONS WITH LONG TIME CONSTANTS Primary Examiner-Benjamin DobeckAttorney-Christensen, Sanborn & Matthews ABSTRACT: A system forproviding continuous and proportional control of a process variableincludes a sensor furnishing an output signal having an analog valueproportional to the process variable. An analog controller includes ashaping circuit comparing that analog value with a reference to developan error signal, and an integrator and two differentiators in series.The time constant of the integrator is chosen to filter out fast,unwanted fluctuations in the process variable and the time constants ofthe differentiators to approximate those in- 16 Claims, 5 Drawing Figs.

volved in the process work function. The outputs of these ele- [52] U.S.Cl 318/590, mum f the analog controller are summed and supplied as a318/257 318/596 control signal to an actuator control circuit whichvaries the [5 1] Int. Cl GOSh 11/18 Power li d t an actuator 50 as t till control a [50] Field of Search 318/590, Conn-0| parameter f theProcess In mm, changes in the 0,257 trol parameter bring the processvariable back to its desired 56 I cud value. An embodiment of the analogcontroller uses tantalum 1 e "megs I capacitors and high impedanceoperational amplifiers. Em I UNITED STATES PATENTS bodiments of theactuator control circuits for use with AC 2,777,285 1/1957 McDonald3l8/596 X synchronous stepping motors and DC motors include an in-2,796,569 6/1957 McDonald et al 318/596X tegrator which eliminates anydead band in the actuator's 2,905,877 9/1959 Ciscel 318/596 X responseto the control signal.

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INDUSTRIAL PROCESS CONTROL SYSTEM FOR OPERATIONS WITH LONG TIMECONSTANTS BACKGROUND OF THE INVENTION This invention relates to controlsystems for industrial processes, and more particularly, to such controlsystems which can provide a critically damped control of processes whichinclude a step or steps having very long time constants.

In industrial process controls, there is generally a variable of theprocess which is monitored to provide an input signal to a controlsystem which in turn acts to maintain that variable at a preset value orto maintain changes in that variable in accordance with a predeterminedschedule. In most industrial processes, there is also a parameterassociated with the process which can be controlled by an output signalfrom the control system to effect changes in the monitored or measuredprocess variable. The manner in which the process variable responds tochanges in the process control parameter is known as the process workfunction and includes both the control function needed to control theprocess variable and other relations peculiar to the process. This workfunction may be expressed in tenns of a mathematical relationshipbetween the process variable and the process control parameter. The timerequired for changes in the control parameter to be reflected in theprocess variable varies with the work function and is known as the cycletime of the process step, or, more simply, as the process time constant.As a simple example, one may consider that the temperature in a largeoven used in a baking step of a process is to be controlled withincertain limits about a desired value. Accordingly, the process variableis temperature and the control parameter may be the amount of powersupplied to the heating coil for the oven.

Control systems have long been known which satisfactorily providecontrol of processes or process steps having relatively short timeconstants. For such processes or process steps, control of the processvariable has been effected by both analog and digital circuits whichprovide stepwise, nonlinear, or proportional control thereof by means ofchanges in the process control parameter.

However, the problems of'obtaining accurate and stable operations of acontrol system when long time constants are involved in the industrialprocess are different. l-leretofore, the control systems known and usedin the industry have not been able to provide proportional control ofthe process variable by means of corresponding control of the processcontrol parameter in the processes having long time constants, with oneexception.

The great majority of the prior systems have been strictly mechanical ora combination of mechanical and electromechanical elements. For example,the level of a product stored within a bin can be controlled by varyingthe flow rate of discharge therefrom. Such a control system would sensethe level of the product in the bin, or the process variable, and applyan appropriate control signal to a valve, for instance, which wouldmodify the flow rate of discharge, or the process control parameter.Previously, the bin level has been sensed by a series of load cellswitches which provide a corresponding number of signals correspondingto different weights of material in the bin, or by a simple on-otfswitch arrangement actuated when the product reached a c certain level.Over a relatively long period of time, the systems maintain the binlevel at or near a desired value. However, because of their mechanicalnature, the type of control is stepwise and discontinuous. In modern,automated process controls, a continuous and generally proportional flowof materials throughout the process is required for successful operationand discontinuous operation of a process step therein cannot betolerated.

One approach in the prior art for providing continuous control has beento adapt digital circuitry to the control system. Such devices, commonlyknown as process control computers, can and do provide continuousproportional control of any process step or subprocess. However, thesecomputers are necessarily expensive to install and expensive tomaintain. Ac-

cordingly, their use is not economically justified for many applicationsin industry in which a relatively small investment has been placed intothe process equipment. Therefore, these types of continuous controlshave not been widely used in those applications.

It is therefor an object of this invention to provide a control systemwhich operates to maintain a control parameter of an industrial processor process step at a predetermined value or in accordance with apredetermined schedule, where the industrial process has a long timeconstant in one or more of its process steps.

It is a further object of this invention to provide such a con trolsystem which is inexpensive and simple and which yet provides continuousand proportional control of the process variable.

It is yet another object of this invention to provide such a controlsystem which operates continuously and proportionally, but also in acritically damped fashion, not withstanding the fact that long timeconstants are involved in the process step or process to which it isapplied.

It is another object of this invention to implement such a controlsystem by means of analog components.

SUMMARY OF THE INVENTION The aforementioned objects and others areachieved, briefly, by a control system which comprises a sensor whoseoutput is directly related to the actual value of the process variable,an analog controller which compares the sensor output with a desiredvalue of the process variable to produce an error signal, and whichfurther operates on that error signal with control circuitry embodyingthe work function and accompanying time constant of the process toprovide therefrom a control signal, and an actuator control circuitresponsive to the control signal for providing an output signal to anactuator regulating a process control parameter, the actuator controlcircuit being able to respond to small values of the control signal fromthe analog controller so as to effect changes in the process controlparameter which are proportionally related to changes in the controlsignal.

BRIEF DESCRIPTION OF THE DRAWINGS The invention can perhaps be bestunderstood by reference to the following description takenin conjunctionwith the accompanying drawings in which:

FIG. 1 is a block diagram of the control system of this invention;

FIG. 2 is a schematic diagram of one embodiment of the analog controllerof FIG. 1;

FIG. 3 is a schematic diagram of one embodiment of the actuator controlcircuit of FIG. 1;

FIG. 4 is a schematic diagram of another embodiment of the actuatorcontrol circuit of FIG. I; and

FIG. 5 is a timing diagram illustrating the operation of the circuit ofFIG. 4.

DESCRIPTION OF A PREFERRED EMBODIMENT With particular reference now toFIG. 1, an industrial process includes a process variable and a processcontrol parameter. It will be recognized by those skilled in the artthat the process variable and process control parameter may beassociated with a single step of the process, with different steps ofthe process, or with all the steps of the process. For example, theprocess variable in the first category may be the level of particulatematerial within the bin, and the process control parameter, the flowrate of discharge from that bin. In the second case, the processvariable may again be the level of particulate material in the bin, butthe process control parameter may be the speed of a conveying meanswhich is located at some distance from the discharge exit or the inletof the bin. In the latter case, the process variable may be the processinput, and the process control parameter may be the process output.

In all of these situations, it is desirable that the process variable bemaintained constant or varied in accordance with a schedule. Inaddition, changes in the process variable can be efiected by changes inthe process control parameter. Finally, the processes underconsideration, as more fully detailed hereinafter, have relatively longtime constants, that is, the time period or cycle required for changesin the process control parameter to be reflected in changes in theprocess variable.

Accordingly, the control system of this invention senses the actualstate of the process variable and provides therefrom an appropriatecontrolling signal to the process control parame ter, which controllingsignal is related to the deviation of the process variable from adesired value and which incorporates therein the work function of theprocess or process step to bring the process variable to its desiredvalue in a critically damped fashion.

More specifically, the actual value of the process variable is sensed bya sensor which produces an output signal A which is generallyproportional to the sensed value. Therefore, the information contentincluding magnitude and sense of the signal A is generally an analog ofthe process variable.

The output signal A is connected to the input of an analog controller 20which operates to produce a control signal B when the process variabledeviates from a desired value and which operates to restore the processvariable to that desired value by means of changes in the processcontrol parameter. Analog controller 20 first includes a shaping circuit22 which has as its inputs the signal A and a signal Z representing thedesired value of the process variable. The function of shaping circuit22 is threefold: First, any nonlinearities in the signal A are minimizedso that signal A is proportional to the actual value of the processvariable. Second, the signal A is amplified to a standard level usablewith the remaining portions of analog controller 20. Third, a comparisonis provided with the desired value of the process variable and an errorsignal proportional to the difference therebetween is producedtherefrom.

In the embodiment of FIG. 1, these three functions are pro vided byappropriate linearizing elements and by level shifting the value of thesignal A. The output signal from shaping circuit 22 is represented asK,A', where A'=AZ, and K a constant.

The signal K,A' is supplied to the input of an integrator 24 and tononinverting input of a summing amplifier 26 upon whose output terminal,the control signal B appears. As will be discussed in more detailhereinafter, integrator 24 may not be required in all applications, forits function is to remove from the error signal K A fluctuations in theprocess variable which have a shorter cycle time than the time constantsof the process or process step which can be controlled through theprocess control parameter but which fluctuations would tend to mask orobscure true or long time changes in the process variable.

The output from integrator 24 is supplied to a differentiator 28 and toan inverting input of the summing amplifier 26. The function ofdifferentiator 28 is to introduce into the control function a cycle timewhich is approximately equal to the time constant of the process orprocess step. In this manner, the system is responsive to the true ordesired signal K,A' which indicates that controlling action is necessaryand which in turn acts to operate on the process control parameter in amanner so that the process variable can be brought back to its desiredvalue.

.The output of differentiator 28 is connected to a second differentiator30 and to a second inverting input of summing amplifier 26. The outputof differentiator 30 is connected to the third inverting input ofsumming amplifier 26.

As described in more detail hereinafter, the function of differentiator30 is to provide the second derivative of the signal K,A so that thecontrol of the process variable may proceed in a critically dampedfashion.

In short, differentiators 28 and 30 approximate the time constantinvolved in the work function of the process or process step.

The summation of the outputs from shaping circuit 22, in-

- tegrator 24 and differentiators 28 and 30 in summing amplifier 26provides the control signal B which represents, in analog form, adesired value for the process control parameter. This desired value issuch as to bring the process variable back to its desired value in acritically damped fashion as detennined by the work function of theprocess or process step associated therewith.

Control signal B is fed to an actuator control circuit 32 which providestherefrom an actuator control signal C which is coupled to an actuator34 for the process control parameter. Under most conditions, the controlsystem including sensor 10 and analog controller 20 operates to maintainthe process variable within very narrow limits of the desired valuethereof. In such cases, the control signal B obtained from the analogcontroller 20 has. an analog value which is generally a small fractionof the maximum control signal therefrom. Accordingly, actuator control32 must be responsive to these small signal levels and yet be able toprovide control of actuator 34 so as to smoothly and continuously varythe process control parameter to bring the process variable back to itsdesired level. In short, actuator control 32 includes components thatapproximate the relatively short time constant or delay involved ineffecting changes in the process control parameter by means of actuator34, so that there is effectively no dead zone or region of nonoperationabout the desired value of the process control parameter. In theembodiment to be hereinafter described, actuator 34 comprises a motor,and actuator control 32 a motor control circuit therefor.

To best understand the operation of the control system just describedand the requirements for the functional components thereof and theirconnections, the types of processes or process steps with which thecontrol system may be used should be examined in some detail.

These processes conveniently may be analyzed in terms of their workfunction and more particularly in terms of the mathematical relationbetween the process control parameter and the process variable. First,the process control parameter may be directly proportional to theprocess variable. Second, the process control parameter may beproportional to the first derivative of the process variable. Third, theprocess control parameter may be proportional to the second derivativeof the process variable. Finally, the process control parameter isrelated by a complicated mathematical relation to the process variable,including combinations of the first three cases.

An example of the first case is a process step in which the speed of amotor actuator is desired to be directly controlled. In such a case, themeasured speed would be the process variable and the input current tothe motor would be the process control parameter. Control systemsadapted for use with industrial process steps of this type are wellknown to the art, and it is felt that the applicability of the controlsystem of this invention lies primarily with processes of the remainingtypes.

One example of the second case would be the control of flow rate throughan orifice in which a pinch valve is used as the flow regulatingelement. Such devices are well known to the art and comprise acompressible, elongated tube whose cross-sectional area is varied by apinching mechanism which compresses the tube in response to the degreeand type of movement of an actuator therefor.

With such valves, the flow rate R is proportional to the cross-sectionalarea of the tube, with R equaling zero when the tube is closed andequaling R when the tube is completely open. It is assumed that the flowrate is also linearly proportional to the degree of compression, asrepresented by the position of the pinching mechanism, a relationbetween the I control signal B supplied to the actuator and the flowrate can be derived. in such a case, the position of the pinchingmechanism is given by the integral of the control signal B. Since theflow rate R is assumed to be linearly proportional to the position r, itcan thus be seen that the flow rate R is also determined by the integralof the control signal B Therefore, the control signal B, or the processcontrol parameter, is proportional to the first derivative of the fiowrate R, or the process variable.

Another example of the second case is the control of temperature in alarge industrial oven. In such ovens, temperature of the oven isgenerally considered as the process variable and the amount of powersupplied to the heating element therefore is generally considered as theprocess control parameter. Because of the long time involved for thetemperature to reach an equilibrium position after a change in the powersupplied to the heating element, it has heretofore not been possible tomeasure the temperature in the oven and to use that measurement as adirect input for the control system. Because these times often reachone-half hour in duration, it has been the practice in some instances tobuild a smaller cavity within the oven which has a shorter time constantand thus a faster response between changes in the power supplied to theheating element and changes in the oven temperature. Again, thetemperature is equal to the integral of the power supplied to theheating element, and thus the power supplied to the heating element, orthe process control parameter, is directly proportional to the firstderivative of the temperature, or the process variable.

An example of the third case would be control of the level ofparticulate material in a bin by means of a pinch valve which controlsthe flow rate discharge therefrom. As previously indicated, the controlsignal supplied to an actuator for the pinch valve is proportional tothe first derivative of the flow rate therethrough. It is also knownthat the level of particulate material within the bin is directlyproportional to the volume of the material within the bin. However, thisvolume is the integral over time of the flow rate of dischargetherefrom. Therefore, the signal supplied to the actuator through thepinch valve, or the process control parameter, is the second derivativeof the bin level, or the process variable.

In all of these cases, it is desirable to obtain stable control of theprocess variable. To insure such control, it has been found that thecontrol system must be sensitive to the rate of change of the processvariable. Therefore, for the first type of process noted above, thecontrol system would have to beresponsive to both the sum of the actualvalue of the process variable plus the first derivative thereof. Forprocesses of the second type, the system would have to be responsive tothe sum of the process variable, plus the first and second derivativesthereof.

Balanced against this requirement are those considerations whichindicate that the electronic circuits used in the control system cannotprovide true integration and differentiation in the mathematical sense.For example, capacitors used in both integrators and differentiatorsleak some charge over a relatively long time period and therefore theircharacteristics are not constant as a function of applied voltage.

Because of these considerations, it has not been found practical todesign a control system for processes or process steps having long timeconstants from a mathematical standpoint alone. Rather, empirical testsshow that for cases where the process control parameter is related tothe second or higher derivative of the process variable, stableoperation can be provided by making the control system responsive onlyto the sensed value of a deviation from the desired process variableplus the first and second derivatives thereof.

Therefore, a control system configured as in the embodiment of FIG. Ican provide stable and critically damped control of most industrialprocesses or process steps having long time constants. As can be seen,the control signal 8 is equal to:

so that the control function of the analog controller 20 approximatesthe work function of the process or process step so that a criticallydamped operation is obtained. Choice of these coefficients can be madeempirically. it is desirable that the analog controller 20 be able toreturn the process variable to its desired value as soon as possible butwith no hunting, jitter, or other undesirable oscillations. The phrasecritically damped" indicates that such control is achieved with only oneoscillation of the process variable about its desired value, and thecoefiicients should be chosen with this criterion in mind.

Reference will now be made to specific embodiments of the analogcontroller 20, the actuator control 32, and actuator 34 in order thatthe invention may be more fully understood.

In FIG. 2, the output signal A from sensor 10 is applied across an inputresistor R of the shaping circuit 22. The voltage developed thereacrossis coupled by a limiting resistor R, to the input of an operationalamplifier All which is referenced to ground potential and whichadditionally has as an input a voltage corresponding to the desiredvalue of the process variable. This voltage is obtained through aresistor R, from the tap of a potentiometer R connected between plus andminus temtinals of the supply voltage V,,. A third input to operationalamplifier A1 is provided by a feedback path from the output thereofincluding the parallel connection of a capacitor Cl and series-connectedresistors RE and R2. The output signal of shaping circuit 22 isdeveloped across potentiometer R3 connected from the output of amplifierA1 to ground potential and a portion thereof, or k A', appears on thetap of potentiometer R3 and is coupled by a resistor R22 to thenoninverting input of an operational amplifier A3 which is referenced toground potential by a resistor R13.

Sensor Ml may comprise any one of a plurality of difierent sensors, suchas thermocouples, capacitive probes, resistive bridges, or the like.What is required is that the signal A be related by some analog to theactual value of the process variable. Resistors R, and R match theimpedance presented to the input terminal to the input impedance ofoperational amplifier Al. The setting of potentiometer R provides a biasvoltage to the input of operational amplifier Al so that the operationalrange of the control system is centered at the desired value of theprocess variable. In the embodiment of FIG. 2, the process variablemaintained at or near a predetermined value by the control system.However, it is also contemplated that the process variable be changed inaccordance with a schedule or in response to another control signal ofan adjacent process step or an adjacent process, and therefore the biassupplied to the input of operational amplifier AI could be provided by avarying voltage source.

Variable resistor R2 is used in the feedback loop along with resistor R1and capacitor C1 to lincarize the output signal from amplifier All overthe desired range of control operation. In addition, the parallelresistance-capacitance combination in the feedback path may be designedto have a relatively short time constant so as to filter out stray noisevoltages that may appear on the output of sensor it).

The output voltage obtained from shaping circuit 22, and moreparticularly, that which is present at the output terminal ofoperational amplifier A1, is thus normalized so that it has a referencevalue when the process variable is at its desired value and has positiveand negative values when the process variable is above or below itsdesired value.

Integrator 24 includes a resistor R4 which couples the output ofoperational amplifier Al to the input of an operational amplifier A2which is referenced to ground potential. A resistor R5 and capacitor C2are connected in a feedback loop around operational amplifier A2 and theoutput terminal thereof is connected to ground potential by apotentiometer R6. The tap of potentiometer R6 is in turn connected to aninverting input of operational amplifier A3 by a resistor R1 1.

As mentioned previously, it is desirable in some applications to removerelatively short term fluctuations in the level of the signal fromshaping circuit 22 so as to make the control system responsive only tolong term or true changes in the process variable. If these fluctuationswere not removed, the control signal 8 would continuously vary betweenmaximum and minimum limit positions, resulting in undesirable hunting,jitter, and oscillatory movements of the actuator 34. On the other hand,the integrating circuit must have a time constant which is shorter thanthat of the process and that of differentiators 28 and 30. An integratorsuch as circuit 24 can cause instability in a control system because thephase of control signals are retarded thereby and therefore the errorsignal I(,A' is increased instead of reduced for the time period ofintegrator operation.

On the assumption that the fluctuations in the error signal K A fromshaping circuit 22 will have a period of less than l minute, and furtherassuming that the output voltage from amplifier A2 has an amplitudewhich is percent of the amplitude of the output of amplifier Al,integrator 24 may be constructed according to the values in table I.

The output terminal of operational amplifier A2 is connected todifierentiator 28 which comprises a capacitor C3 in series with apotentiometer R7. The common point of capacitor C3 and potentiometer R7is connected to the diflerentiator 30 which comprises a capacitor C4 inseries with a potentiometer R8. The taps of potentiometers R7 and R8 arecoupled to the inverting input of operational amplifier A3 by resistorsR10 and R9, respectively.

Capacitors C3 and C4, and resistors R7 and R8 have equal values so thatthe approximate time constants or cycles thereof are equal.Representative maximum cycles or time constants for various values ofthe difierentiator components can be seen from table II.

It will be noted that these times vary from ten minutes to ninetyminutes. To obtain such long constants with simple circuits such asillustrated in FIG. 2, the leakage of charge from the capacitors C3 andC4 must be controlled.

In a working model, C3 and C4 each comprised tantalum electrolyticcapacitors. For use with dual polarity signals such as would beencountered in the circuitry of FIG. 2, two of these electrolyticcapacitors were connected in a back-toback fashion. The tantalumcapacitor has a very small leakage current when charged and can berecharged after current reversal by a minimal charging current of lessthan I pa. In contrast, an aluminum electrolytic capacitor requires aconsiderably larger initial current to develop such isolation. Moreover,the use of operational amplifiers with very highinput impedances alsominimizes the leakage of charge from the capacitors. In the workingmodel, the operational arnplifiers comprised integrated circuits havinginput impedance in the order of a few megohms. I

If time constants longer than 90 minutes are desired, the capacitors andoperational amplifiers must be chosen more carefully. However, it ispossible with devices currently on the market to obtain suitablecombinations of high-input impedance and low-capacitor leakage currentsso as to extend the time constants. w

The output signals from shaping circuit 22, integrator 24, anddiflerentiators 28 and 30 are summed'in operational amplifier A3 whichalso includes a resistor and acapacitor in the feedback loop thereof soas to filter out noise signals from the output thereof, which is coupledthrough a resistor R15 to actuator control circuit 32.

The setting of the taps of potentiometers R3, R6, R7, and R8 determinesthe coefficients K,, K,, K,, and K in the control function for theanalog controller heretofore described. These coefficients must be setso that the control function is critically damped and provides a controlsignal B which in fact returns the process variable to its desired valueas quickly as possible without hunting or oscillation.

A working model was constructed and the component types and values usedtherein are listed in table III.

The entire analog controller of FIG. 2 was assembled on a single printedcircuit board and provided control accuracies in the range of 0.1 to 1percent.

The analog value of the control signal B varies from plus to minus V forthe extreme desired values of the process control parameter, and equalsreference potential for no change therein.

The function of actuator control circuit 32 is to convert the desiredchange in the process control parameter represented by the controlsignal B into a control signal C for the actuator 34. In almost allcases, the control signal C can be represented by the amount of AC OR DCpower supplied by actuator control circuit 32 to actuator 34.

For example, when the actuator 34 is an AC operated device, such as asynchronous or an induction motor, a heating element for an oven, or thelike, control is effected by varying the number of cycles of AC powerthat are supplied to the actuator. In the case of AC motor operatedvalves, such control results in an adjustment in the direction and speedof operation of the motor so that the valve is moved to intermediate,maximum, or minimum positions thereof. In the case of AC motor operatedconveyors, such control results in an adjustment of the speed ofrotation of the conveying means. In the case of a heating element for anoven, such control results in an adjustment of the heat deliveredthereby. In the cases where there is a limiting position, such as in thecase of an AC motor operated valve, the actuator control circuit mustalso include limit switches to stop the operation of the actuatorcontrol circuit 32 when the limit positions are reached.

Where the actuator 34 is a DC operated device, control is effected byvarying the duty-cycle of the DC power supplied thereto. in the casewhere the actuator 34 comprises a DC motor operated valve, for example,such control results in an adjustment in the direction and speed ofoperation of the DC motor so that the valve varies between maximum,minimum, and intermediate positions.

There are a number of criteria which must be satisfied in order to havea stable control. First, the actuator control circuit 32 must be able tovary the power delivered to the actuator from maximum to minimum values,and in some cases do so bidirectionally. Secondly, the actuator controlcircuit 32 must be able to modify the amount of power delivered betweenthese values with very high resolution. When industrial processes havinglong time constants are to be controlled, it is necessary for stableoperation that the control signal B be provided for very smalldeviations of the process variable from the desired value. It is thusessential that the actuator control circuit 32 have a resolution so asto respond to these very small values and accordingly control the amountof power supplied to actuator 34. Third, it is desirable that theactuator 34 be able to change its speed, direction of movement orrotation, or other output in a manner which is proportionally related tothe control signal B.

One embodiment of an actuator control circuit which can satisfy thesethree requirements is illustrated in FIG. 3. This circuit is designedfor use with an AC Slo-Syn motor actuator, which is a bidirectionaldevice, but could as easily be applied to the control of nondirectionalactuators such as a heating element.

A Slo-Syn motor includes an armature having a plurality of pole pairsand first and second field windings therefor. When AC power is appliedacross one of the field windings, the armature rotates in incrementalsteps, the degrees of rotation of each step depending on the number ofpole pairs in the annature. In addition, the direction of rotation isdetermined by which of the field windings is energized, and the speed ofrotation is synchronous with the applied AC voltage, i.e., the armaturerotates one step for every cycle of the AC voltage.

Because of these properties, the Slo-Syn motor is very adaptable to thebidirectional control of the position of a valve which in turn is usedto vary the flow rate of material therethrough.

When the shaft of the Slo-Syn motor is coupled to the valve by asuitable gear reduction mechanism, the actuator may have a resolutionper step on the order of 10 the full excursion of the valve from maximumto minimum positions. For use with the control system of this invention,it is sufficient that the resolution be on the order of 0.1 to 1 percentof full excursion so that the motor can be controlled in largerincrements of 10 to 100 steps.

To this end, the circuit in FIG. 3 includes an integrator to which thecontrol signal B is coupled and which provides an output signal to firstand second control circuits, one for each direction of Slo-Syn motoractuation. The output from each of these control circuits controls theactuation of a bidirectional, controllable semiconductor device which isconnected in series with one of the field windings of the Slo-Syn motoracross an AC voltage source. in addition, a feedback connection isprovided from the output of each control circuit to the input of theintegrator to accordingly control the time period of actuation of eachbidirectional controllable, semiconductor device.

-In more detail, the control signal B from analog controller 20 isapplied through a limiting resistor R16 to the inverting input of anoperational amplifier A4. A capacitor C5 is connected in the feedbackloop of operational amplifier A4, and the output thereof is connected tothe inputs of the first and second control circuits.

The first control circuit includes transistors 01, Q3, Q5, andbidirectional, controllable semiconductor device, or Triac, 'lRl. Theoutput of operational amplifier A4, or the output of the integrator, iscoupled to the base electrode of transistor Q1 by a resistor R19. Thebase-to-emitter junction of transistor O1 is shunted by a diode D1, andthe emitter of transistor Q1 is coupled to ground potential. Thecollector of transistor 01 is connected to the supply voltage V by abiasing resistor R23 and is also connected to the base of transistor 03by a resistor R25. In turn, the emitter of transistor O3 is coupled toground potential, and the collector thereof to the supply voltage V by aresistor R29. A second input to transistor 01 is provided by a resistorR27 coupled to the collector of transistor 03 and a third input theretois provided by a resistor R21 connected to the negative supply voltage-V,.

A second input to operational amplifier A4is provided by a feedbackresistor R17 connected to the collector of transistor Q3. The collectorof transistor O3 is also connected to the base of transistor Q5 by aresistor R3]. in turn, the collector of transistor O5 is coupled to thesupply voltage V, by biasing resistor R33 and the emitter thereof isconnected directly to the gate electrode of Triac TRl. A field coil L1of the Slo-Syn actuator is connected at one end to a source of volts ACand at the other end, through appropriate limit switches LS1 and LS2, toa main current-carrying terminal of the Triac TRl. The other mainterminal of Triac TRl is connected to ground potential.

Now, the input signal B has maximum and minimum values of :V,. Fornormally balanced operations, however, the signal 8 does not usuallyexceed 10.1 V, and generally fluctuates within this range. The functionof the integrator is to filter out normally balanced changes of thismagnitude from appearing as firing signals to the Triac TRl but yet toallow the control unit to be responsive to much smaller, unbalancedcontrol signals. In most cases, the design should allow for response tounbalanced input signals smaller than 0.01 V which last over arelatively long time interval.

Assuming now that a very small, negative polarity input signal B hasbeen applied to resistor R16, the output of operational amplifier A4rises from zero or ground potential to the potential of the supplyvoltage V, at a rate determined by the time constant of the R16-C5combination and the magnitude of the input signal. in the case of verysmall input signals, this integration may take several minutes. As longas the output of operational amplifier A4 is near reference potential,the current applied to transistor Q1 by resistor R19 is very small.Accordingly, resistor R21 connected to the negative supply voltage V,maintains transistor Qi in a nonconducting state and diode D1 preventsan excessive reverse-bias on the base to emitter-junction thereof.

With transistor O1 in a nonconducting state, transistor Q3 is biased byresistors R23 and R25 in a conducting state. Accordingly, the voltage atthe collector of transistor O3 is very low, on the order of 0.02 V, to0.05 V,. At this time, transistor Q5, and thus Triac TR1, are maintainedin a nonconducting state by virtue of the connection through resistorR31 from the collector of transistor Q3. At the same time, the currentfeedback from the collector of transistor O3 to the base of transistorQ1 via resistor R27 is insufficient to place transistor Q1 in aconducting state.

When the small, negative polarity input signal is integrated, the outputvoltage of operational amplifier A4 rises to the supply voltage V,. Whenthat voltage approaches approximately 0.8 V., the current throughresistor R19 is sufiicient to offset the bias provided by resistor R21and accordingly transistor 01 is placed in a conducting state. At thistime, the voltage at the collector thereof drops to 0.02 V, to 0.05 V,.Accordingly, transistor 03 is placed in a nonconducting state so thatits collector voltage rises to approximately 0.9 V,. At this time,transistor O1 is maintained in a conducting state via the feedback loopincluding resistor R27 and in addition transistor Q5 is placed in theconducting state by the connection through resistor R31. When transistorQ5 turns on, current is supplied through the emitter thereof to the gateof Triac TRl to place it in the conducting state so that a current pathis completed through field winding L1 from the AC source to groundpotential.

At this time, the Slo-Syn motor begins to rotate in the directiondetermined by the polarity of field winding L1. The number of cycles ofAC power supplied to field winding L1 is determined by the feedback pathincluding resistor R17. When transistor O3 is placed in the conductingstate, a positive voltage is applied across resistor R17 to supplycurrent to the input of operational amplifier A4 which tends to offsetthe negative input signal supplied through resistor R16. Therefore, theoutput voltage of operational amplifier A4 drops at a rate determined bythe time constant of the R17-C5 combination. During the time when theoutput voltage of operational amplifier A4 is decreasing, transistor Q1is maintained in a conducting state by the feedback loop includingresistor R27. However, when the output voltage of operational amplifierA4 drops to approximately 0.1 V,, the current through resistors R19 andthe feedback current through resistor R27 are not sufficient to offsetthe bias supplied through resistor R21 and therefore transistor O1 isplaced again in a nonconducting state. Similarly, transistor Q3 isplaced in a conducting state and transistor Q5 and Triac TR1 in anonconducting state.

At such a time, power is removed from the field winding L1, and theSlo-Syn motor ceases to rotate.

With this circuit, the minimum time in which power is supplied to thefield winding Ll of the Slo-Syn motor, or the minimum on time, isdetermined by the magnitude of the voltage applied across resistor R17and the time constant of the Rl7-C5 combination. This time constantcannot be made shorter than one cycle of the AC voltage which isavailable to drive the Slo-Syn motor, for it must be remembered thatsuch motors make one step for every cycle of the applied voltagethereacross. It is preferable that this time constant provide aresolution which is equal to the control accuracy of the entire controlsystem, which, as stated before, is approximately 1 percent. Therefore,the time constant of the R17-C5 combination should not be longer than 1percent of the time constant involved in the process or process step, orthat embodied in the time constant of the analog controller 20 used indifferentiators 28 and 30. In such a case, the minimum on" time of thecontrol circuit results in perhaps 10 to 100 steps of the Slo-Synactuator which, through suitable gear reduction, could be made toprovide a desired minimum position change in the control valve.

For larger magnitudes of input signal, the relation of the "on and "off"times changes, as the on" times become longer and the off times becomeshorter. At approximately 0.8 V,, in the case of negative polarity inputsignals, the Triac TRl is continuously energized and the Slo-Syn motoraccordingly driven continuously in one direction. The value of the inputresistor R16 must therefore be chosen so that for a maximum value of thecontrol signal B, the Triac TRl is continuously energized. In such acase, the current supplied by the feedback resistor R17 is notsufficient to offset the signal supplied from resistor R16 so that theoutput of the operational amplifier A4 remains at the maximum value.

The control circuit works in an identical manner for positive polaritycontrol signals B, with the exception that transistors Q2, Q4, Q6,forming a part of the second control circuit along with their associatedresistive components, actuate Triac TR2 which is connected in serieswith the other field winding L2 of the Slo-Syn motor.

Capacitor C6 and resistor l R35 connected across field windings L1 andL2 function as a phase-shifiing network. Limit switch LS1 is providedfor overriding manual operation of the Slo-Syn motor and provides forbypassing Triacs TRl and TR2 when it is desired to rotate the Slo-Synmotor in either of the two directions. Limit switch LS2 is responsive tothe position of the Slo-Syn motor shaft and operates to break theconnection to ground through either Triacs TRl or TR2, or limit switchLS1 when the Slo-Syn shaft is driven to one of its extreme positions.

A working model of the circuit in FIG. 3 was constructed using thecomponent types and values illustrated in table lV.

TABLE IV Component Type and Value R16 l M R17 1 M R" 1 M M9 150 K11 R20150 K11 R21 220 xn R22 220 K11 R23 l0 xn R24 10 K0 R25 500 um R26 500 K0R27 150 K0 R28 150 um R2! 10 K1! R30 10 k0 R31 I00 kn R32 I00 k0 R33 1kn R34 1 kn R35 250 n C5 I pf, double pole tantalum electrolytic C6 3.3t. D1 IN 462 D2 IN 462 ()1 2N5223 Q2 2N5223 Q3 2N5223 Q4 2N5227 Q52N5227 Q6 2N5227 TRl RCA 40529 TR2 RCA 40529 A4 CA-3033 (RCA) or As withthe analog controller 20, it can be noted that the capacitor C5comprises a double pole tantalum electrolytic capacitor and theoperational amplifier A4 a high impedance integrated circuit so thatleakage of charge from the capacitor CS was minimized.

When the actuator 34 comprises a DC motor, for example, the embodimentof actuator control circuit 32 shown in FIG. 4 may be used. The basicelements of this circuit are similar to those in the embodiment of FIG.3 and comprise an integrator, first and second control circuits coupledthereto which provide an output signal denoting the desired period ofconduction of a bidirectional controllable semiconductor device orTriac, the DC motor, and a limit switch in circuit with the DC motor andthe bidirectional controllable semiconductor device. In this instance,however, only a single Triac is actuated by the output signals from thefirst or second control circuits to apply power across the DC motor forselected portions of alternate half-cycles of an AC voltage source. i

As with the AC actuator control circuit of FIG. 3, the control signal Bfrom analog controller 20 is first connected to an integrator. In FIG.4, this integrator includes a resistor R36 which couples thecontrolsignal B to the inverting input of an operational amplifier A5which is referenced to ground potential. Capacitor C7 is connected inthe feedback loop around operational amplifier A5, and the output ofoperational amplifier A5 is coupled by a resistor R40 to the commonjunction of a capacitor C10 and first and second diodes D3 and D4. Theother terminal of capacitor C10 is connected to ground potential. Thiscommon junction serves as the input to both the first and second controlcircuits in F IG. 4.

A second input to the operational amplifier A5 is provided by a feedbackvoltage from the DC motor 34. In particular, the motor includes anarmature 34a whose one terminal is connected to one side of a volt ACsource and whose other terminal is connected to ground potential througha limit switch LS3 and a Triac TR3. The common junction between TriacTR3 and limit switch LS3 provides a wellthe speed of the armature and isconnected by -a resistor R37 to the inverting input of operationalamplifier A5.

The circuit of FIG. 4 provides speed control of the DC motor 34 byapplying appropriate triggering pulses to Triac TR3 at desired points onalternate half-cycles of the l20 volt AC waveform across armature 34a..These triggering pulses are derived by the first and second controlcircuits in which a level-shifted, phase-shifted AC waveform is comparedin magnitude with the integrated control signal appearing at the commonjunction between C10 and D3, D4.

The following description will consider only the first control circuit,as the second control circuit is identical thereto.

The level-shifted, phase-shifted AC waveform is derived from the 120volt AC source by a voltage divider network including resistors R38 andR41 and a capacitor C9 which are connected in series between the 120volt AC source and the negative supply voltage -V,. The voltage thusappearing at the common junction of resistor R41 and capacitor C9 is asinusoidal waveform. For purposes of explanation only, this waveform hasa peak-to-peak voltage variation of :3 volts. In addition, a resistorR43 is connected from the common junction of resistor R41 and capacitorC9 to the negative voltage supply V, and adds to this sinusoidalwaveform a negative bias of approximately 4 volts. This common junctionis coupled directly to the control electrode of a transistor O7 in thefirst control circuit.

Therefore, the voltage at the base of transistor 07 is a sinusoidalwaveform which is delayed by 90 in phase from the AC wavefonn suppliedacross armature 34a, and which has a maximum peak value of -l volt and aminimum peak value of 7 volts. Plots of the waveforms across armature34a and the voltage V, supplied to the base electrode of transistor 07can be seen in FIGS. 5a and 5b, respectively.

The emitter of transistor Q7 is connected to diode D3 and the collectorthereof is coupled by a capacitor C11 to the base of a transistor Q9. Inaddition, resistor R44 connects the collector of transistor Q7 to thevoltage supply V, and resistor R46, diode D5, and capacitor C13 couplethe base of transistor 09 to V,. Finally, the emitter of transistor O9is connected directly to V,.

The collector of transistor Q9 serves as the output of the first controlcircuit and is connected by a resistor R48 to the gate electrode ofTriac TR3.

With particular reference now to FIG. 5, the firstcontrol circuit isdesigned to actuate the Triac TR3 during positive polarities of thevoltage waveform V applied across armature 34a. 1

When control signal B is zero, the output of operational amplifier A5 isat reference potential. In this situation, the voltage at the emitter oftransistor 07 is also atground potential. In the embodiment of FIG. 4,transistor 07 is a PNP-type and therefore requires a signal polarity onits base which is relatively positive with respect to that on itsemitter in order for the transistor to be placed in a conducting state.It can therefore be seen that if the voltage at the emitter thereof isat ground potential, transistor O7 is maintained in a nonconductingstate because the voltage at the base thereof, as represented by thecurve in FIG. 5b, has a maximum value of -l volt.

When the control signal B has some positive magnitude, the output of theintegrator. appearing as the output of operational amplifier A5 risesgradually in a manner similar to that previously described for theintegrator of FIG. 3. With a positive control signal B, the output ofoperational amplifier A5 gradually becomes more negative. Accordingly,the voltage at the common junction of diode D3 and capacitor Cdecreases. Transistor Q7 is then triggered into a conducting state whenthe voltage on the emitter thereof decreases to a value less than thevoltage onthe base thereof minus the baseto-emitter drop, or V Thevoltage on the emitter of O7 is voltage across capacitor C10, or V010.In FIG. 5b, this point occurs at 3 volts and transistor 07 is placed inthe conducting state at I.

At this time, capacitor C11 is provided with a charge path through'transistor Q7, diode D3 and capacitor C10 to ground potential. Theresultant charging pulse provides a current to the base of transistor 09which places that transistor in a conducting state. At this time, acurrent pulse is developed in resistor R48, as best illustrated in FIG.5d, and applied to the gate of Triac TRB to place it in a conductingstate. Power is then supplied to the annature 34a for the remainder ofthe positive half-cycle, as best illustrated by the shaded portion ofFIG. 5a. Transistor Q7 remains in a conducting state until the voltageat theibase thereof decreases below -3 volts. How ever, capacitor C11 isdischarging and therefore transistor 09 is turned off. Therefore, TriacTR3 is placed in a nonconducting state when the voltage V,,,.,,,thereacross passes through its zero crossover point.

Speed regulation is achieved through the back EMF voltage from armature34a. This voltage always has a polarity which is opposite to the senseof the control signal B. Therefore, the signal supplied to the input ofoperational amplifier A5 is gradually decreased until a regulatedoperating point is achieved, due to the feedback connection through R37.1f the DC motor is controlling a position device, such as a valve, thevoltage on the output of operational amplifier A5 is gradually reducedto zero as the device nears its desired position and the processvariable is returned to its desired value. If the DC motor iscontrolling a continuously operating device, such as a conveyor, theoutput voltage from the operational amplifier A5 is reduced to a stablevalue.

Diode D3 blocks reverse breakdown current through the base-to-emitterjunction of transistor 07 when the reverse bias voltage thereacrossexceeds a certain magnitude, e.g., 5 volts. This condition will occurwhen the output of operational amplifier A5 has a positive polarity.Diode D5 provides a discharge path for capacitor C11 when transistors Q7and 09 are placed in a nonconducting state. Resistor R46 and capacitorC13 block the generation of high frequencies by transistor 07 and Q9through parasitic feedback loops.

The second control circuit includes transistors Q8 and Q10 and operatesin an identical manner to actuate Triac TR3 during negative half-cyclesof the voltage waveform V,,,.,,,. Accordingly, transistor 08 responds topositive output signals from operational amplifier A5. Waveformsillustrating the operation of the second control circuit can be seen inFIGS. 50 and Se, in which Triac TR3 is fired at the crossover point II.

By comparing curves 5b and 5c, it can be seen that there is a dead zoneof approximately 2 volts, which extends from +1 volt to l volt, in whichneither of the control circuits respond to output signals fromoperational amplifier A5. This dead zone is necessary to prevent theTriac TR3 from being fired on both half-cycles of a single cycle of thewaveform VR or to prevent firing thereof by the positive polarity, firstcontrol circuit during negative half-cycles, and vice versa.

The dead zone at the input to the control circuits is necessary becauseit is very difficult to accurately maintain the voltages at the basesand emitters of transistors Q7 and Q8 so that there is no overlap. Thelevel of these voltages are always subject to thermal drift in thecircuit components and additionally depends to some extent upon theaccuracy of the component values. If normal components with normaltolerances are to be used in the design, it is necessary to have such adead zone.

However, the overall actuator control circuit 32 has no dead zone. Thatis, the integrator including operational amplifier A5 responds to verysmall unbalanced values of the control signal B and eventually providestherefrom an output voltage which has a magnitude sufficient to triggerone of the transistors Q7 or Q8. This point occurs when the voltageacross capacitor C10, plus the base-to-emitter drop of either transistorQ7 or 08, plus the forward voltage drop of diodes equal to the forwardvoltage drop of diode D3, or V,, plus the D3 or D4, exceeds :l volt.

As a result, the circuit in FIG. 4 is well suited for use with processesor process steps having relatively long time constants. The timeconstant of the integrator represented by the R36-C7 combination shouldbe selected for a value less than 1 percent of the process and systemtime constant.

The remaining components of the actuator control circuit in FIG. 4include a full-wave rectifier which comprises diodes D9, D10, D11 andD12 connected in a bridge configuration to supply DC power to a fieldwinding L3 of the DC motor 34 from the 120 volt AC source. Where the DCmotor is to be used to set the position of the variable setting device,such as a pinch valve, a limit switch LS3 may be interposed in theconnection between armature 34a and Triac TR3. Limit switch LS3 includesthe parallel connection of two series branches comprising diodes D7 andD8 and two pairs of limit switch contacts. Normally, the limit switchcontacts are closed and power alternately flows through the seriesbranches on successive half-cycles of the voltage V,,,,,,. When themotor reaches one of its limit positions, the corresponding limit switchcontacts open to break the current path through the corresponding seriesbranch. Thus, if the DC motor 34 were running in a forward direction andthe polarity of the voltage V,,,.,, was positive, the limit switchcontacts in series with diode D7 would open. The other series branchincluding diode D8 thereafter provides a discharge path for thetransients generated upon the opening of the limit switch contacts.

A working model of the circuit in FIG. 4 was constructed and includedthe component types and values listed in table V.

TABLE V Component Type and Value R36 1 M R37 12 M R38 I ki'l R39 20 k0RM 2 kfl R41 20 kn R41 33 kt! R43 33 R0 R44 I00 kn R45 I00 k0 R46 22 k0R47 22 k0 R48 I00 (I R49 100 n C! l 12, double pole.

tantalum electrolytic C8 I if. C9 I "I. ClO 1 t1, double pole tantalumelectrolytic C11 0.05 n. C12 0.05 ,n'. C13 800 pF. CM 800 pF. D3 IN 462D4 IN 462 D IN 462 D6 IN 462 D7 200 V 2A power D8 200 V 2A power D9 D200 V IA bridge Dll rectifier DIZ Q7 2N3904 QB 2N3906 Q 2N440l Q102N4403 TR3 40526 (RCA) A5 (IA-3033 (RCA) In this model, the velocity ofthe DC motor was controlled from approximately 0.2 to 1,700 r.p.m., thatis, over-a range of nearly 1-:10.

As briefly discussed heretofore, the DC actuator control circuit whichis taught in FIG. 3 can be used with practically any AC actuator. Wherethe actuator can provide bidirectional feeds back a voltage which has apolarity opposite to that of the control signal B. in both of thesecircuits, the time constant of the feedback resistor and capacitorcombination should be chosen to make the resolution of the actuatorcontrol circuit, or the minimum ON" time of the AC control circuit,equal to control the process control system. in addition, the value ofthe input or limiting resistor must be chosen so that for the maximumvalue of the control signal B, the actuator is continuously energized.This value may be chosen from the following relation:

where R equals the value of the input resistor, R, equals the value ofthe feedback resistor, V equals the maximum value of the control signalB, and V,- equals the magnitude of the feedback voltage.

While this invention has been described in terms of a number ofpreferred embodiments thereof, it is to be clearly understood by thoseskilled in the art that the invention is not limited thereto, but ratheris bounded only by the limits of the appended claims.

What is claimed is: l. A system for use with a process including aprocess variable and a process control parameter which is related tosaid process variable by a defined work function, said work functionincluding a relatively long time constant, comprising:

a. a sensor furnishing an output signal which is proportionally relatedto the actual value of said process variable, b. an analog controllerincluding i. first means comparing said output signal with a desiredvalue therefor and producing an error signal proportional to anydifference therebetween, and

ii. second means operating on said error signal to produce a controlsignal which varies in accordance with said work function,

c. an actuator controlling said process control parameter in response toan input signal, and

d. an actuator control circuit which is responsive to small values ofsaid control signal to produce said input signal for said actuator.

2. A system as recited in claim 1, wherein said second means of saidanalog controller operates on said error signal with the work function11/! (FA B=K|A +K;1"dT+K 35',

where B the analog value of said control signal, A the analog value ofsaid error signal, and K K; and K are constants whose values are chosenso that changes in said process control parameter return said processvariable to its desired value in a critically damped manner.

3. A system as recited in claim 2, wherein said second means comprises:

a. a first differentiating circuit producing from said error signal afirst derivative signal,

b. a second differentiating circuit producing from said first derivativesignal a second derivative signal, said first and said seconddifferentiating circuit each having a time constant equal to that of theprocess work function, and

c. means summing said error signal, said first derivative signal, andsaid second derivative signal to produce said control signal.

operation, the embodiment of FIG. 3 may be used without any 4. A systemas recited in claim 3, wherein:

a. each of said first and second differentiating circuits comprises abipolar tantalum capacitor and a resistor connected in series, and

b. said summing means comprises an operational amplifier having a veryhigh input impedance.

5. A system as recited in claim 1, wherein said second means of saidanalog controller operates on said error signal with the work functionwhere A equals the analog value of said error signal, B equals theanalog value of said control signal, and K,, K K and K, are constantswhose values are chosen so that changes in said process controlparameter return said process variable to its desired value in acritically damped manner.

6. A system as recited in claim 5, wherein said second means comprises:

a. an integrating circuit producing from said error signal an integralsignal,

b. a first differentiating circuit producing from said integral signal afirst derivative signal,

c. a second differentiating circuit producing from said first derivativesignal a second derivative signal, the time constants of said first andsaid second differentiating circuits being equal to that of the processwork function and the time constant of said integrating circuit beingcloser to filer undesired short-term variations from said error signaland being no longer than percent of the time constants of the processwork function, and

d. means summing said error signal, said integral signal, said firstderivative signal, and said second derivative signal to produce saidcontrol signal.

7. A system as recited in claim 6 wherein:

a. each of said first and said second differentiating circuits comprisesa bipolar tantalum capacitor and a resistor connected in series,

b. said summing means comprises an operational amplifier having a veryhigh input impedance, and

c. said integrating circuit comprises an operational amplifier and abipolar tantalum capacitor, said capacitor being connected in a feedbackloop around said operational amplifier.

8. A system as recited in claim 1, wherein said actuator control circuitcomprises:

a. an integrator including an operational amplifier having input andoutput tenninals and a very high input impedance, a capacitor connectedin a feedback loop between said input and said output terminals, aninput resistor coupling said control signal to said input terminal, anda feedback resistor having one side thereof connected to said inputterminal,

b. a control circuit having input and output terminals, said inputterminal being connected to the output terminal of said operationalamplifier, said control circuit being operative to provide a pulse onits output terminal when the signal on said input terminal thereofexceeds a predetermined value,

c. an AC voltage source,

d. a bidirectional, controllable semiconductor means having a pair ofmain, current-carrying terminals and a control terminal and operative toallow current between said main terminals only for the portion of ahalf-cycle of an AC waveform which follows the application of a controlsignal to said control terminal,

e. means connecting said main terminals of said bidirectional,controllable semiconductor means in series with said actuator acrosssaid AC voltage source,

f. means connecting said output terminal of said control circuit to saidcontrol tenninal of said semiconductor means,

g. means developing a feedback voltage whose polarity is opposite tothat of said control signal,

h. means connecting said feedback voltage to the other side of saidfeedback resistor, the time constant of said feedback resistor and saidfeedback capacitor being chosen to determine the minimum period ofconduction of said bidirectional, controllable semiconductor means andthe values of said feedback voltage and said input resistor being chosenso that for the maximum expected value of the control signal, saidbidirectional, controllable semiconductor means is continuouslyconducting.

9. A circuit for providing continuous control of an AC actuator whichincludes a control element, in response to a control signal without anydeadband, comprising:

a. an integrator including an operational amplifier having input andoutput terminals and a very high input impedance, a capacitor connectedin a feedback loop between said input and said output terminals, aninput resistor coupling the control signal to said input terminal, and afeedback resistor having one side thereof connected to said inputtenninal, said integrator being operative to provide an integral signalfrom said control signal whose slope is determined by the time constantof said input resistor and said feedback capacitor,

b. a first control circuit having input and output terminals, said inputterminal being coupled to said output terminal of said operationalamplifier, said first control circuit being operative to first providean output terminal whose polarity opposes that of said integral signalwhen said integral signal exceeds a first value which is less than itsmaximum expected value, said first control circuit ceasing to providesaid output signal when said integral signal decreases below a second,lower value,

c. a first bidirectional, controllable semiconductor means which has apair of main, current-carrying terminals and a control terminal andwhich is operative to allow current between said main tenninals only forthe portions of a half-cycle of an applied AC wavefonn which follows theapplication of a signal to said control terminal,

. an AC voltage source,

e. means connecting said main terminals of said first bidirectional,controllable semiconductor means in series with the control element ofsaid actuator across said AC voltage source,

f. means coupling said output terminal of said first control circuit tosaid control terminal of said first bidirectional, controllablesemiconductor means, and

g. means connecting the other side of said feedback resistor to saidoutput terminal of said first control circuit.

10. A circuit as recited inclaim 9, wherein said feedback capacitorcomprising a tantalum capacitor and said first bidirectional,controllable semiconductor means comprises a Triac.

11. A circuit as recited in claim 9,. for use with an AC actuatorcomprising a bidirectional synchronous stepping motor having first andsecond field windings, and wherein the control signal variesproportionall between positive and negative maximum values for desiredenergizations of the first and second field windings, respectively:

a. wherein, said operational amplifier inverts the polarity of thecontrol signal applied thereto and wherefor said first control circuitresponds to positive values of said integral signal and saidbidirectional, controllable semiconductor means is connected in serieswith the second field winding of the actuator, and

b. further comprising 1. a second control circuit having input andoutput terminals, said input terminal being coupled to said outputterminal of said operational amplifier, said second control circuitbeing operative to first provide an output signal on its output terminalwhose polarity opposes that of said integral signal when said integralsignal exceeds a predetermined negative value which is less than itsmaximum negative value, said second control circuit ceasing to providesaid output signal when said integral signal decreases below a second,lower negative value thereof,

2. a second bidirectional, controllable semiconductor means having apair of main, current-carrying terminals and a control'terminal,

3. means connecting said main terminals of said second bidirectional,controllable semiconductor means in series with the first field windingof said actuator across said AC source,

4. means coupling the output terminal of said second control circuit tosaid control terminal of said second bidirectional, controllablesemiconductor means, and

5. a second feedback resistor connected from said output terminal ofsaid second control circuit to said input terminal of said operationalamplifier.

12. A circuit as recited in claim 11, wherein said feedback capacitorcomprises a tantalum capacitor and said first and second bidirectional,controllable semiconductor means comprise 'l'riacs.

13. The system as recited in claim 12, further comprising first andsecond limit switches interposed between said first and second fieldwindings and said second and first Triacs, respectively.

14. A circuit for providing bidirectional speed control of a DC motorwhich includes an armature from an AC source, in response to a controlsignal without any deadband, comprismg:

a. an integrator including an operational amplifier having input andoutput terminals and a very high-input impedance, a capacitor connectedin a feedback loop between said input and said output terminals, aninput resistor coupling the control signal to said input terminal, and afeedback resistor having one side thereof connected to said inputterminal, said integrator being operative to provide an integral signalfrom the control signal whose polarity is opposite thereto and whoseslope is determined by the time constant of said input resistor and saidfeedback capacitor,

b. an AC voltage source including one terminal which defines a referencelevel thereof,

c. first means developing from said AC voltage source a first waveformwhose phase is delayed therefrom and whose level is always below that ofsaid reference level, second means developing from said AC voltagesource a second waveform whose phase is delayed 90 therefrom and whoselevel is always above that of said reference level,

e. third means providing a first trigger pulse whenever the level ofsaid first waveform exceeds that of said integral signal,

f. fourth means providing a second trigger pulse whenever the level ofsaid second waveform is less than that of said integral signal,

g. a bidirectional, controllable semiconductor means having a pair ofmain, current-carrying terminals and a control terminal which isoperable to allow current between said main terminals only for theportion of a half-cycle of an applied AC waveform which follows theapplication of a pulse to said control terminal,

h. means connecting said main terminals of said bidirectional,controllable semiconductor means in series with the armature of the DCmotor across said AC voltage source,

i. means coupling said first and said second trigger pulses to saidcontrol terminal of said bidirectional, controllable semiconductordevice, and

j. means connecting the common junction of the armature of the DC motorand the main terminals of said bidirectional, controllable semiconductormeans to the other side of said feedback resistor.

15. A circuit as recited in claim 14, wherein said feedback capacitorcomprises a tantalum capacitor and said bidirectional, controllablesemiconductor means comprises a Triac.

16. A circuit as recited in claim 14, further including a limit switchinterposed in the connection between the annature and saidbidirectional, controllable semiconductor means, said limit switchcomprising two series branches, each series branch including a diode anda pair of limit switch contacts, wherein said diodes are oppositelypoled.

PMM UNITED STATES PATENT OFFICE 5 CERTIFICATE OF CORRECTION Patent No. 3,628 ,ll'6 Dated December 14, 1971 Inventor(s) Fritz K. Preikschat It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

In the Specification: 8

Col. l, line 61, after "a" (first occurrence) delete c Col. 4, line 68,after "open" insert If -'and change "It" to it Col. 6, line 28, delete"k A'" and insert therefor K A' Col. 8, line 60, delete "OR" and inserttherefor or Col. 15, Table V, adjacent Component R48, "1009" should beunder Type and Value column.

Col. 16, line 15, after "t d lete "control" and insert therefor theminimum resolution of Col. 10, lines 49, "Qi" should be Q Signed andsealed this. 20th dayof June 1972.

(SEAL) Attest:

EDWARD M.FLETCHER, JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents 1?" UNTTED STATES PATENT OFFICE CERTTTICATE QT QGRRECTTONPaten 3 3 ,628 ,ll6' Dated December 14 1971 Inven:or(s) Fritz K.Preikschat It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

In the Specification:

Col. 1, line 61, after "a" (first occurrence) delete c Col. 4, line 68,after "open" insert If 'and change "It" to it Col. 6, line 28, delete "kA'" and insert therefor K A' Col. 8, line 60, delete "OR" and inserttherefor or Col. 15, Table V, adjacent Component R48, "1009" should beunder Type and Value column.

Col. 16, line 15, after "to" delete "control" and insert therefor theminimum resolution of Col. 10, lines 49 "Qi" should be Q Signed andsealed this: 20th day of June I972.

(SEAL) Attest:

EDWARD M.FLETGHER, JR ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

1. A system for use with a process including a process variable and aprocess control parameter which is related to said process variable by adefined work function, said work function including a relatively longtime constant, comprising: a. a sensor furnishing an output signal whichis proportionally related to the actual value of said process variable,b. an analog controller including i. first means comparing said outputsignal with a desired value therefor and producing an error signalproportional to any difference therebetween, and ii. second meansoperating on said error signal to produce a control signal which variesin accordance with said work function, c. an actuator controlling saidprocess control parameter in response to an input signal, and d. anactuator control circuit which is responsive to small values of saidcontrol signal to produce said input signal for said actuator.
 2. Asystem as recited in claim 1, wherein said second means of said analogcontroller operates on said error signal with the work function where Bthe analog value of said control signal, A'' the analog value of saiderror signal, and K1, K3 and K4 are constants whose values are chosen sothat changes in said process control parameter return said processvariable to its desired value in a critically damped manner.
 2. a secondbidirectional, controllable semiconductor means having a pair of mAin,current-carrying terminals and a control terminal,
 3. means connectingsaid main terminals of said second bidirectional, controllablesemiconductor means in series with the first field winding of saidactuator across said AC source,
 3. A system as recited in claim 2,wherein said second means comprises: a. a first differentiating circuitproducing from said error signal a first derivative signal, b. a seconddifferentiating circuit producinG from said first derivative signal asecond derivative signal, said first and said second differentiatingcircuit each having a time constant equal to that of the process workfunction, and c. means summing said error signal, said first derivativesignal, and said second derivative signal to produce said controlsignal.
 4. A system as recited in claim 3, wherein: a. each of saidfirst and second differentiating circuits comprises a bipolar tantalumcapacitor and a resistor connected in series, and b. said summing meanscomprises an operational amplifier having a very high input impedance.4. means coupling the output terminal of said second control circuit tosaid control terminal of said second bidirectional, controllablesemiconductor means, and
 5. a second feedback resistor connected fromsaid output terminal of said second control circuit to said inputterminal of said operational amplifier.
 5. A system as recited in claim1, wherein said second means of said analog controller operates on saiderror signal with the work function where A'' equals the analog value ofsaid error signal, B equals the analog value of said control signal, andK1, K2, K3 and K4 are constants whose values are chosen so that changesin said process control parameter return said process variable to itsdesired value in a critically damped manner.
 6. A system as recited inclaim 5, wherein said second means comprises: a. an integrating circuitproducing from said error signal an integral signal, b. a firstdifferentiating circuit producing from said integral signal a firstderivative signal, c. a second differentiating circuit producing fromsaid first derivative signal a second derivative signal, the timeconstants of said first and said second differentiating circuits beingequal to that of the process work function and the time constant of saidintegrating circuit being closer to filer undesired short-termvariations from said error signal and being no longer than 10 percent ofthe time constants of the process work function, and d. means summingsaid error signal, said integral signal, said first derivative signal,and said second derivative signal to produce said control signal.
 7. Asystem as recited in claim 6 wherein: a. each of said first and saidsecond differentiating circuits comprises a bipolar tantalum capacitorand a resistor connected in series, b. said summing means comprises anoperational amplifier having a very high input impedance, and c. saidintegrating circuit comprises an operational amplifier and a bipolartantalum capacitor, said capacitor being connected in a feedback looparound said operational amplifier.
 8. A system as recited in claim 1,wherein said actuator control circuit comprises: a. an integratorincluding an operational amplifier having input and output terminals anda very high input impedance, a capacitor connected in a feedback loopbetween said input and said output terminals, an input resistor couplingsaid control signal to said input terminal, and a feedback resistorhaving one side thereof connected to said input terminal, b. a controlcircuit having input and output terminals, said input terminal beingconnected to the output terminal of said operational amplifier, saidcontrol circuit being operative to provide a pulse on its outputterminal when the signal on said input terminal thereof exceeds apredetermined value, c. an AC voltage source, d. a bidirectional,controllable semiconductor means having a pair of main, current-carryingterminals and a control terminal and operative to allow current betweensaid main terminals only for the portion of a half-cycle of an ACwaveform which follows the application of a control signal to saidcontrol terminal, e. means connecting said main terminals of saidbidirectional, controllable semiconductor means in series with saidactuator across said AC voltage source, f. means connecting said outputterminal of said control circuit to said control terminal of saidsemiconductor means, g. means developing a feedback voltage whosepolarity is opposite to that of said control signal, h. means connectingsaid feedback voltage to the other siDe of said feedback resistor, thetime constant of said feedback resistor and said feedback capacitorbeing chosen to determine the minimum period of conduction of saidbidirectional, controllable semiconductor means and the values of saidfeedback voltage and said input resistor being chosen so that for themaximum expected value of the control signal, said bidirectional,controllable semiconductor means is continuously conducting.
 9. Acircuit for providing continuous control of an AC actuator whichincludes a control element, in response to a control signal without anydeadband, comprising: a. an integrator including an operationalamplifier having input and output terminals and a very high inputimpedance, a capacitor connected in a feedback loop between said inputand said output terminals, an input resistor coupling the control signalto said input terminal, and a feedback resistor having one side thereofconnected to said input terminal, said integrator being operative toprovide an integral signal from said control signal whose slope isdetermined by the time constant of said input resistor and said feedbackcapacitor, b. a first control circuit having input and output terminals,said input terminal being coupled to said output terminal of saidoperational amplifier, said first control circuit being operative tofirst provide an output terminal whose polarity opposes that of saidintegral signal when said integral signal exceeds a first value which isless than its maximum expected value, said first control circuit ceasingto provide said output signal when said integral signal decreases belowa second, lower value, c. a first bidirectional, controllablesemiconductor means which has a pair of main, current-carrying terminalsand a control terminal and which is operative to allow current betweensaid main terminals only for the portions of a half-cycle of an appliedAC waveform which follows the application of a signal to said controlterminal, d. an AC voltage source, e. means connecting said mainterminals of said first bidirectional, controllable semiconductor meansin series with the control element of said actuator across said ACvoltage source, f. means coupling said output terminal of said firstcontrol circuit to said control terminal of said first bidirectional,controllable semiconductor means, and g. means connecting the other sideof said feedback resistor to said output terminal of said first controlcircuit.
 10. A circuit as recited in claim 9, wherein said feedbackcapacitor comprising a tantalum capacitor and said first bidirectional,controllable semiconductor means comprises a Triac.
 11. A circuit asrecited in claim 9, for use with an AC actuator comprising abidirectional synchronous stepping motor having first and second fieldwindings, and wherein the control signal varies proportionally betweenpositive and negative maximum values for desired energizations of thefirst and second field windings, respectively: a. wherein, saidoperational amplifier inverts the polarity of the control signal appliedthereto and wherefor said first control circuit responds to positivevalues of said integral signal and said bidirectional, controllablesemiconductor means is connected in series with the second field windingof the actuator, and b. further comprising
 12. A circuit as recited inclaim 11, wherein said feedback capacitor comprises a tantalum capacitorand said first and second bidirectional, controllable semiconductormeans comprise Triacs.
 13. The system as recited in claim 12, furthercomprising first and second limit switches interposed between said firstand second field windings and said second and first Triacs,respectively.
 14. A circuit for providing bidirectional speed control ofa DC motor which includes an armature from an AC source, in response toa control signal without any deadband, comprising: a. an integratorincluding an operational amplifier having input and output terminals anda very high-input impedance, a capacitor connected in a feedback loopbetween said input and said output terminals, an input resistor couplingthe control signal to said input terminal, and a feedback resistorhaving one side thereof connected to said input terminal, saidintegrator being operative to provide an integral signal from thecontrol signal whose polarity is opposite thereto and whose slope isdetermined by the time constant of said input resistor and said feedbackcapacitor, b. an AC voltage source including one terminal which definesa reference level thereof, c. first means developing from said ACvoltage source a first waveform whose phase is delayed 90* therefrom andwhose level is always below that of said reference level, d. secondmeans developing from said AC voltage source a second waveform whosephase is delayed 90* therefrom and whose level is always above that ofsaid reference level, e. third means providing a first trigger pulsewhenever the level of said first waveform exceeds that of said integralsignal, f. fourth means providing a second trigger pulse whenever thelevel of said second waveform is less than that of said integral signal,g. a bidirectional, controllable semiconductor means having a pair ofmain, current-carrying terminals and a control terminal which isoperable to allow current between said main terminals only for theportion of a half-cycle of an applied AC waveform which follows theapplication of a pulse to said control terminal, h. means connectingsaid main terminals of said bidirectional, controllable semiconductormeans in series with the armature of the DC motor across said AC voltagesource, i. means coupling said first and said second trigger pulses tosaid control terminal of said bidirectional, controllable semiconductordevice, and j. means connecting the common junction of the armature ofthe DC motor and the main terminals of said bidirectional, controllablesemiconductor means to the other side of said feedback resistor.
 15. Acircuit as recited in claim 14, wherein said feedback capacitorcomprises a tantalum capacitor and said bidirectional, controllablesemiconductor means comprises a Triac.
 16. A circuit as recited in claim14, further including a limit switch interposed in the connectionbetween the armature and said bidirectional, controllable semiconductormeans, said limit switch comprising two series branches, each seriesbranch including a diode and a pair of limit switch contacts, whereinsaid diodes are oppositely poled.